In optical communication systems, photodetectors are used to convert optical signals into electrical signals. The most commonly used photodetectors are positive-intrinsic-negative (p-i-n) photodiodes and avalanche photodiodes.
A typical p-i-n photodiode includes an absorption layer of intrinsic, i.e. not intentionally doped, semiconductor material between a region of extrinsic, i.e. doped, semiconductor material of a first conductivity type, i.e. n-type or p-type, and a region of extrinsic semiconductor material of a second conductivity type, i.e. p-type or n-type, an arrangement that produces an electric field in the absorption layer. In operation in photoconductive mode, a reverse voltage is applied to the p-i-n photodiode to enhance the electric field in the absorption layer. Light incident on the p-i-n photodiode is absorbed by the absorption layer to generate current carriers, i.e. electrons and holes, in an absorption process. The generated current carriers are separated by the electric field in the absorption layer and drift toward the regions of extrinsic semiconductor material: holes drift toward the region of p-type semiconductor material, and electrons drift toward the region of n-type semiconductor material. The resulting photocurrent is proportional to the optical power of the incident light.
A typical avalanche photodiode includes a multiplication layer of intrinsic or lightly doped extrinsic semiconductor material, in addition to an absorption layer of intrinsic semiconductor material, between a region of extrinsic semiconductor material of the first conductivity type and a region of extrinsic semiconductor material of the second conductivity type, an arrangement that produces electric fields in the multiplication layer and the absorption layer. In operation, a large reverse voltage is applied to the avalanche photodiode to enhance the electric fields in the absorption layer and the multiplication layer. As in the p-i-n photodiode, light incident on the avalanche photodiode is absorbed by the absorption layer to generate current carriers, in an absorption process. The generated current carriers are separated by the electric field in the absorption layer, such that either holes or electrons drift toward the multiplication layer. The electric field in the multiplication layer is large enough that the holes or electrons acquire sufficient kinetic energy to generate additional current carriers through impact ionization. The generated current carriers, in turn, generate additional current carriers through impact ionization. Thus, current carriers are multiplied in an avalanche multiplication process in the multiplication layer, leading to a multiplied photocurrent.
One general concern particularly in long-haul receiver applications is the problem of reliability of photodiodes working at optical telecommunications wavelengths of about 1.3 and 1.5 microns. In order to detect light at these wavelengths, a narrow-bandgap absorption layer material, such as InGaAs, is used. It is very important not to expose a narrow-bandgap material to an external environment, especially in photodetectors used in long haul, high data rate applications. Even though effort has been invested in the development of surface-passivation techniques intended to reduce the surface defects and traps, nonetheless, many photodetector types, mesa-based photodetectors in particular, have not yet been able to deliver satisfactory performance to pass stringent Telcordia aging tests. Therefore, it is an object of the present invention to be able to manufacture photodiodes without exposing the narrow-bandgap photon absorption layer, or, preferably, InGaAs layer.
Another concern, which is related to both reliability and overall performance of avalanche photodiodes, is that of containing high electric fields within the field-charge and impact multiplication layers. Device modeling shows the electric field within these layers remains at a very high value and extends further away from the photon detection area, which can cause photodiode characteristics to drift with time.
The traditional methods of terminating the narrow-bandgap photon absorption layer(s) and/or the electric field within the field-charge layer and impact multiplication layer for p-i-n photodetectors and avalanche photodetectors fall into one of three categories:
1) Only exposing sensitive layers to air upon wafer sawing or cleaving into chips. The majority of the one-top-contact diffusion p-i-n photodiodes are fabricated this way worldwide. The narrow-bandgap photon absorption layer retains its integrity throughout the whole device area. The majority of one-top-contact (no n-well) InP/InGaAs and InAlAs/InGaAs avalanche photodiodes are also fabricated this way.
2) Only exposing sensitive layers during trench or mesa etch, with subsequent passivation of the exposed areas by one of the following: a) epitaxial regrowth and b) plasma-enhanced chemical vapor deposition (PECVD) or sputtering dielectric film(s) such as SiNx or SiO2, or c) spin-coating polyimide or a BCB film.
3) Passivating sensitive areas by ion implantation within the mesa surface or within the etched trenches.
Among all these practices in the prior art, only diffusion-created field termination and surface passivation can deliver satisfactory reliability performance to meet stringent data communications and telecommunications requirements. In the present invention, the electric field within the field-charge and impact multiplication layers can be terminated at a predetermined distance away from the recess window by using a method which is most closely related to the 3rd method above. Moreover, an additional electrical capacity reduction can be obtained by using the method of present invention.
One accepted measure of performance of a photodetector is a gain-bandwidth product. This is usually expressed as gain figure in dB multiplied by the detector bandwidth in GHz. In order to make a high-performance avalanche photodiode, one has to maximize the photocurrent multiplication while minimizing the overall device capacitance.
In order to maximize the photocurrent multiplication, one can increase the electric field across the multiplication layer. Unfortunately, increasing electric field can cause another problem to occur. Unwanted fringe electric fields often appear at the edges of electrode(s) of the photodiode, which may lead to an uncontrollable, ongoing avalanche process. In order to reduce the fringe fields, the upper electrode, i.e. the electrode facing incoming light, is often made in a form of a ring on top of the photosensitive area of the photodiode, and additional field-control ring structures are sometimes implemented. These additional field-control ring structures can increase capacitance of the avalanche photodiode.
In operation, the ring structure of the upper electrode blocks some of the photosensitive area and thereby increases requirements on optical alignment of the elements intended to deliver the light to the photodiode. Of course the ring can be made larger; but this leads to increased capacitance which further reduces the gain-bandwidth product and, therefore, reduces the benefit of having a higher photocurrent multiplication coefficient in the first place. Thus, a higher gain of an avalanche photodiode is offset by increased capacitance and reduced bandwidth due to larger electrode structure.
A number of methods have been suggested in prior art which are aimed at reduction of the photodiode capacitance and control of the fringe fields. In one method, a mesa is etched and passivated on the side of the semiconductor stack which is opposite to the ring electrode side, such that the electric field is confined to the un-etched area of the semiconductor stack. Other methods and respective devices rely on etching a concave portion in the multiplication layer of semiconductor stack, with a dopant diffusion being limited to the concave portion using photolithographically produced masks.
While to some extent being capable of limiting fringe electric fields, the methods of prior art are complex and expensive due to the increased number of diffusion steps required and also photolithographic steps required to limit dopant diffusion to the concave area only. Furthermore, the prior art methods expose the sensitive layers to environment at some point in the manufacturing process, which is detrimental to overall device reliability as has been described above. Finally, in some prior art devices, the ohmic ring contact blocks some light which otherwise could be utilized to produce photocarriers.
The present invention can be used to effectively control the fringe fields in an avalanche photodiode without compromising the gain-bandwidth product or the size of the photosensitive area of the photodiode. In addition, in the method of present invention, the sensitive layers of the semiconductor stack are not exposed to environment after initial deposition of these layers has been completed.